Jupiter

Jupiter holds more than twice the mass of all the other planets combined. How a gas giant is built, what lies beneath its clouds, and how it shaped the rest of the solar system.

Diameter: 142,984 km (11 × Earth)
Mass: 318 × Earth
Gravity at cloud tops: about 2.5 × Earth
Distance from Sun: 5.2 AU (about 778 million km)
Year: 11.9 Earth years
Day: about 9 hours 56 minutes
Moons: more than 90 known
Cloud-top temperature: about −110 °C

The build: grow fast, then grab gas

Jupiter formed beyond the frost line, where ice added a lot of extra solid material. That let it grow a heavy core — perhaps ten or more times Earth's mass — quickly enough to do something rocky planets cannot: pull in huge amounts of hydrogen and helium from the protoplanetary disk before the gas disappeared.

Once a core passes a certain mass, gas capture runs away. Jupiter ended up with about 318 Earth masses, roughly 2.5 times the mass of all the other planets put together. It was probably the first planet to finish forming, within the first few million years.

No surface to stand on

Jupiter is mostly hydrogen and helium. Below the colourful clouds, the gas gets denser and hotter with depth until, under enormous pressure, hydrogen becomes a liquid that conducts electricity like a metal. Currents in this metallic hydrogen generate a magnetic field nearly 20,000 times stronger than Earth's.

NASA's Juno spacecraft, orbiting since 2016, found that Jupiter's core is not a small, sharp ball but appears “dilute” — heavy elements seem to be mixed partway into the surrounding hydrogen. One possible explanation is a giant impact early in Jupiter's history, but this is still being studied.

Weather on a giant scale

Jupiter's fast rotation — a day of under ten hours — stretches its clouds into bands. The Great Red Spot is a storm larger than Earth that has been observed for well over 150 years, although it has been shrinking in recent decades.

A miniature solar system

Jupiter's four largest moons, discovered by Galileo in 1610, are worlds in their own right:

Io — the most volcanically active body in the solar system, heated by tidal squeezing.
Europa — an icy shell over a global salty ocean, one of the best places to look for life beyond Earth. NASA's Europa Clipper launched in 2024 to study it.
Ganymede — the largest moon in the solar system, bigger than Mercury, with its own magnetic field and probably a buried ocean. ESA's JUICE mission launched in 2023 to study it.
Callisto — ancient, heavily cratered and possibly hiding an ocean too.

Jupiter as a builder of other planets

Jupiter's gravity stirred up the asteroid belt and may have prevented a planet from forming there. It scatters comets and asteroids, sending some out of the solar system and some inward. Many models suggest its early movements shaped the inner solar system, possibly including how small Mars turned out and how much water reached Earth.

Lessons for planet builders

To build a gas giant, you need a big core early, before the disk gas is gone.
Giant planets shape the whole system around them.
Some of the most habitable real estate may be on the moons of giants.